Telephone system



4 Sheets-Sheet 1 TELEPHM]-SYSTEM Filed DSC. 20, 1941 s. M. BABcocK ET AL Jan. 19, 1943.-

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TELEPHONE SYSTEM Filed Dec. 20, 1941 4 Sheets-Sheet 2 /NV'ENTORS MALOGAN. .5y I

.imv 19, 1943.

s. M. BABcocK TAL 2,308,637 TELEPHONE, SYSTEM Filed Dac: 20, A1941 4 Slieetg-Sheet 3.

s. M. BAcoc/r MEMO 'M ALOGA/v ATTORNEY Jam. 19, 1943. s. M. BABCOCK ET AL TELEPHONE 4SYSTEM 4 sheet-sheet 4 Filed Dec. 20, 1941 S. M ABcoc/r v @bx Patented Jan. 19, 1943 TELEPHONE SYSTEM Stuart M. Babcock, Chatham, and Mason A. Lo-

gan, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 20, 1941, Serial No. 423,812

15 Claims.

This invention relates to signaling and communication systems and particularly to telephonev systems of the type in which connections between calling and called stations are completed by means, such as automatic switches, controlled fromthe calling station.

Telephone communication systems have been proposed heretofore, in which the telephone designations assigned to telephone lines or trunk circuits are transmitted from one point to another by means of direct current impulses of di'erent polarities, which singly or in combination serve to characterize the different numerals, or symbols of which the designations are composed. A particular system of this character is disclosed in a copending application, Serial No. 412,402, led Sept. 26, 1941. This system contemplates the use of a, rectifier or varistor type of key sender at the subscribers station, a three-phase alternating current supply at the central office, and a signal receiving network or circuit comprising a plurality of differently poled polarized code relays arranged in either delta or Y connections. The code relays, two of which are included in each of the three .legs of the delta or Y terminationy are operated in various combinations in accordance with the actuation of the various keys of the varistor type key sender, to characterize the several digits, or symbols of the telephone designation. The code relays, through the medium of a translator circuit, control the operation of register relays in a decoder register circuit in which the transmitted designations are registered and stored for subsequent use in controlling the operation of automatic switches. The automatic switches, so controlled, function, in a well-known manner, to extend the connection from the call originating station to the called station.

It is the object of this inventionto simplify and improve telephone designation transmission systems of the type disclosed in the above-identified copending application.

This object is attained in accordance with a feature of the invention by utilizing a source of single-phase alternating current of relatively high frequency at the central oiiice in place of the 60 cycle, three-phase supply used in the prior system. This feature results in a simplification and a reduction in cost of the apparatus required, and effects a system having improved capabilities.

Another feature of the invention contemplates terminating the tip and ring conductors of the code receiving circuit at opposite terminals vof a transformer secondary, through which the singlephase alternating current is supplied to the telephone line, and in utilizing, as a third leg', a branch circuit extending between ground and a center tap on the transformer secondary. In,Y

each of the tip and ring leads of the code receiving circuit, and in the said branch circuit there is included a pair of oppositely poled polarized relays, which operate in various combinations to characterize the digits or symbols of a telephone designation.

A further feature of the invention resides in a novel keying arrangement whereby. the digit characterizing codes are obtainable in a simplified manner, and which permits the use of the third, or branch' circuit of the code receiving circuit mentioned above. In accordance with this feature, certain of the digit characterizing codes are obtained by connecting a single varistor from the tip or ring conductor of the line to ground and eifectively short-circuiting the other conductor to ground. By virtue of this arrangement, the direct current produced by the single varistor is caused to ow over the conductor to which it is connected, and to divide and ow over the other, now grounded, conductor, and also over the third or branch leg of the code receiving circuit. In this manner a code relay in each of thethree leads, tip, ring, and third branch, of the code receiving circuit is operated on current whose direction is determined by a single varistor. The term varistor is used in this specification to characterize a dry rectifier of th'e copper-oxide type, or similar device.

A still'further feature of the invention resides in a novel type of supervisory circuit. Any alternating current supervisory scheme encounters the difficulty that the loop capacitance causes a charging current to flow, and this current, to which the supervisory relay is subject, obscures the change of impedance over the loop when the switchhook at a subscribers station is operated, upon which impedance change the supervisory relay depends for its operation or release. This difliculty is minimized, in accordance with this particular feature of the invention, by normally subjecting the supervisory relay to a current vof such phase -and magnitude as to cancel out a portion of the current due to maximum loop capacitance, When the handset telephone is not removed from its cradle support, this current is such that, when combined with the current produced by the loop capacitance which increases with increasing length of subscribers line, the resulting current in the supervisory relay is reduced when the subscribers telephone set is disconnected from a maximum length line, and is increased on a zero length line to a value about the same as the new value obtaining on a maximum length line.

These and other features or" the invention will be readily understood from the following detailed description taken in connection nit the accompanying drawings, in which:

Fig. 1 shows the varistor type key sender located at a calling subscribers station, and also the code signal receiving circuit located at a central oice;

Fig. 2 is a circuit diagram showing the automatic switches which serve to extend a connection from a calling subscribers station to a called subscribers station, and also the translator circuit which is controlled by the code signal receiving circuit of Fig. l;

Fig. 3 is a circuit diagram illustrating the decoder register circuit and associated apparatus which is controlled by the translator circuit of Fig. 2, and which registers and stores the telephone designations transmitted from the calling station of Fig. 1; f

Fig. Y4 is a block diagram showing the manner in which Figs. 1, 2 and 3 are to be assembled to eiect a complete system;

Fig. 5 is a table showing the varistor combinations resulting from the actuation of each ofthe ten keys of the key sender, and the corresponding combination of code relays which respond to each key actuationand l Fig. 6 is a graphical representation of the several currents which are involved in the super visory feature of the invention.

It will be observed that the translator circuit of Fig. 2 and the decoder register circuit of Fig. 3 are substantially identical to the translator and decoder register circuits shown in Figs. 4 and 3, respectively, of the above-identified copending application. The only material change made in these figures is that involving the identication of certain of the multicontact translator relays. Relays MP2, MN?, TF2 and TNZ of the prior application disclosure are identiiied in the present disclosure as relays TF2, TNZ, MP2 and MNE, respectively. In View of the fact that the translator and decoder register circuits have been fully described in the above-referred-to copending application, the preliminary brief description of this equipment is omitted from this application. This equipment, however, will be fully described in the description of the operation of the system to be made presently.

The key sender E at the subscribers station A comprises ten keys a, c, d, e, f, g. h, i, and k. In practice, each key would be equipped with a cap or similar character-bearing device which displays a set of designation symbols corresponding to those appearing on the number-plate of the conventional telephone dial. Key a, would bear the symbol 1; key the symbols ZABC; key c, the symbols SDEF; key d, the symbols 4GHI; key e, the symbols EJKL; key f, the symbols SMNO; key g, the symbols 'YPRS key It, the

symbols STUV; key 5i, the symbols SWXY; and g key It, the symbols OQPERATOR. Some of the keys, Such as keys c, d, 7L and 7 control a single contact whereas others control a plurality of contacts. At the lower left of the key sender K there are shown four dry rectiers, or varistors I, 2, 3 and li. The varistors l and 2 are adapted tolbe connected to the tip side of the line through the operation of the keys, while the varistors 3 and li are adapted to be similarly connected to the ring side of the line. varistors i and 3 function to effect the transmission of positive direct current pulses over the tip and ring sides, respectively, whereas varistors 2 and 4 function to effect the transmission of negative direct current pulses over the tip and ring sides, respectively.

At the central office there is located a singlephase source, P, of alternating current which supplies current to the leads T, R and M through the medium of transformer I0. The frequency of this source is preferably cycles. The lead M is known as the ground lead and extends from the center tap of the secondary winding of transformer Ii) to ground by way of the operating windings of4 code relays MP and MN. The leads T and R extend from opposite end terminals of the transformer secondary and include the operating windings of code relays TN and TP and RN and RP, respectively. These leads, as will appear from a later description, connect with corresponding leads of the line L when a call is initiated at station A. Relays TN, TP, MP, MN, RN and RP are referred to as code relays since they operate n diierent combinations of two or three at a time in accordance with the code set up by the keys of the key-type sender K. A 100 cycle elimination iilter FT, FR, FM, is bridged across each pair of code relays to prei/,ent them from buzzing on alternating current of the fundamental frequency of source P.v rBecause these relays do not operate except when direct current is present, there is no release current requirement imposed on them dueto the alternating current drawn by the loop capacitance or leakage. The supervisory circuit is shown at the lower right of Fig. 1. It comprises a repeating coil RC, a step-down transformer Il, a full-wave rectifier indicated at X, a supervisory relay l2 and an electrolytic condenser I3. The repeating coil RC is connected in the ring lead R at Athe central oice and the impedance looking vinto'it is low enough so as to have very little effect on the currents in the code relays. The resistance of relay I2 is relatively low so that very little direct current potential drop occurs across it. Condenser I3, shunted across the relay I2 provides a low impedance shunt for the alternating current components, thus smoothing the current that ows in the relay and helping to hold down the impedance looking into the repeating coil RC. As will be described more fully hereinafter, the voltage introduced by the step-down transformer II is of such phase and magnitude as tocancel out a portion of the voltage due to loop capacitance, thereby insuring the proper operation of relay I2 in response to receiver switchhook operations.

The table in Fig. 5 illustrates how the varistors L12, 3 and l may be connected in ten diierent combinations to pass ten different signals to the central oice. It is to be understood that the possible varistor combinations are not limited to ten. Only ten have been indicated, since this num-ber is adequate to provide a comprehensive understanding of the arrangement when used in the telephone designation transmission system disclosed in the drawings. As a matter of fact, the table of Fig. 5 shows a particular assignment of only ten of the codes to the ten digits or characters from which a telephone designation is constructed. The arrangement suggested by Fig. 5 is one which lends itself to the relatively simple translator circuit shown in Fig. 2, for translating the operation of the code relays oi` Fig. 1 into signals receivable by the decoder register circuit of Fig. 3.

` The ten diierent codes used in the description of the system disclosed are obtained as follows:v Four codes are obtained by connecting one rectifier at a time. These codes, as shown in the table of Fig. 5 characterize the designation char acters BDEF, 4GHI, 8TUV and QWXY. These codes each bring up two code relays at the central office as indicated in the table. Two more codes are obtained b y connecting two rectiers at a time. These codes, as shown in the table, characterize the designation characters 5JKL, and 0. The last four codes are obtained by connecting three rectiers at a time. These codes, as shown in the table, characterize the designation characters l, 2ABC, 6MNO and 'IPR-S.

.The code relays which operate for each combination of varistors connected are indicated in Fig. 5.

The rst four codes mentioned above cause the operation of two code relays. In the case of the rectier combinations 2-4 and 1 3, corresponding to the designations 5JKL and 0, respectively, three code relays at the central oiice are operated. The currents in the tip and ring relays TN, TP and RN, RP are the same as if single rectiers were connected, and the current in the ground relays MN and MP is double since the currents due to the two rectiflers add together in the ground lead.

The rectifier combinations l-3-4, 2-3-4, 1 2-3, and 1-2-4 (designation characters 1, 2ABC, BMNO, and 7PRS, respectively) are equivalent to connecting a rectifier from one side of the loop to ground and short-circuiting the other side of the loop to ground. With the combination 1 3-4, for example, the direct current is produced by rectifier I, while the others merely serve to short the ring lead to ground. Because of the ring-to-ground short, the direct current which would ow only in the ground lead if No. rectifier were connected alone, is made to divide and flow partly in the ring lead R which is paralleled with it; the result is that three central oce code relays (MP, TP and RN) are operated.

The foregoing discussion explains how it is possible to obtain the ten codes used in the designation transmission system disclosed, with only single phase alternating current.

A detailed description of the operation of the system disclosed in the drawings (Figs. 1, 2 and. 3) will now be made and for this purpose it will be assumed that the subscriber at station A desires a connection with the subscribers station B (Fig. 2), the telephone designation oi which, for descriptive purposes, will be assumed to be ABl-5678. The subscriber at station A accordingly lifts the handset from its support and actuates keys b, c, d, c, f, g, and h in that order.

When the handset at station A is removed from its support, the line circuit is closed, and the line finder F operates, in a well-known Inanner, to seize the calling subscribers line L. At the same time the sender selector S-I becomes eiective to extend the calling line through to an idle sender. When these operations have been completed, a circuit is closed from the alternating current source P at the central oce over the closed subscribers loop at station A by way of the leads T and R, conductors H4 and H3, brushes |0I and |00 of switch S-I, secuenca switch contacts |02 and |03, tip and ring brushes and corresponding terminals of line nder F. and the tip and ring conductors of line L extending to the called station A. The polarized code relays TP andTN, included in the lead- T, relays RP and RN, included in the R lead, and relays MP and MN, includedv in the Mv lead do not operate on this current because lters FT,

FR, and FM eliminate this current from these relays.

Repeating coil RC is included in the ring conductor of the circuit just traced and it introduces the metallic alternating line current into the full-Wave rectifier X in series with the small out-of-phase voltage from transformer |I causing the operation of relay I2, its operating current being rectified by means of the full-wave rectier X.

Relay I2, operated, establishes an obvious circuit for the off-normal sloW-to-release relay 400. Relay 400, at its upper armature and front contact, connects oli-normal ground to the armature of signal present relay 402 by way of conductor 40|. Relay 402 now operates in a circuit extending from grounded battery, upper winding of relay 402, conductor 403, right-handcontacts and armatures of code relays MN and MP, conductor 40|, to ground by way ofthe front contact and upper armature of relay 400.

Relay 402, operated, establishes an obvious operating circuit for enabler relay 404, which, in operating, causes the "signal recorded relay 400 to operate in a circuit which may be traced from grounded battery, lower armature and front contact of relay 400, conductor 405, armature and left-hand contact of relay 404, to ground by way of the operating winding of relay 406. With relays 400, 402, 404 and 400 operated, the translator circuit is prepared to receive the telephone designation digits, or symbols, characterized by the particular code relays that operate.

Upon the actuation of key b at station A in accordance with the first character A of the desired, or assumed, telephone designation, contact Q operates, as it does when each key of the key sender K is actuated. The operation of contact Q insures the opening of one of the leads to the subset so that there is no low resistance path from tip to ring while the signal is being passed. Whenever the off-normal switch Q is closed, the subscriber will hear an audible tone in the receiver due to the alternating current voltage being impressed on the line from the source P. A varistor |05 is connected across the receiver at station A to reduce the receiver noise caused by the operation of switch Q.

When key b is actuated at station A, it will be observed that the right-hand spring, which is connected to the ring conductor of the line by way of the switchhook contacts, closes on contact 20 which connects with varistor 3, while the lefthand spring, which is connected to the tip conductor of the line, closes simultaneously on contacts 2i and 22 which connect respectively to varistors I and 2. The closure of contacts 2| and 22 results in an effective short-circuiting of the tip conductor to ground. Therefore, the varistor 3 alone functions to transmit the controlling direct current impulse over the line. The circuit for this current may be traced from ground, positively poled varistor 3, closed contacts 20 ofkey b, switchhook contacts at station A, ring conductor of the line L,` ring terminal and corresponding brush of finder switch F, sequence switch contacts |03, brush |00 and associated contact of switch S-I, conductor I3, left-hand windings of repeating coil RC, operating windings of code relays RP and RN, lead R, lower half of the secondary winding of transformer to the center tap thereof, and' thence in parallel over two paths, one extending from the said center tap and including lead M and the lower windings of code relays MP and MN, to ground, and the other extending from the said center tap and including the upper half of the secondary of transformer I0, lead T, lower windings of code relays TN and TP, conductor ||4, to ground, it being understood that conductor ||4 is connected to the tip conductor of line L which, as above indicated, is now effectively shorted to ground at the calling station A. The currents in the circuits just traced are in such direction as to cause the operation of polarized code relays MP, TN and RP.

Relay MP, operated, opens the operating circuit for relay 402. Relay 402, released, starts the slow-to-release relay 404 to release; connects the ground on conductor to the armature of relay 406, which ground operates timing relay 408 in a circuit extending from grounded battery, upper winding of relay 408, back contacts and middle upper armatures of relays TPE, TN2, RNZ and RPZ, back contacts and outer upper armatures of relays MP2 and MN2, conductor 403, left contact and armature of relay 400, left contact and armature of relay 402, conductor 40|, to ground at the iront contact and armature of relay 400. By way of conductor |21, which is a continuation of conductor 409, ground is connected to the armatures of code relays RP, RN, TP and TN (Fig. 1).

Since there is no battery Voltage connected to any of multicontact translator relays TP2, TN 2,

RN2, RP2, MP2 and MN2 at this time, none of i these relays operates due to the operation of the code relays MP, TN and RP. When relay 40S operated, it removed ground from the lock and transfer lead |40 extending to the decoder register circuit (Fig. 3), and applied locking ground to the multicontact translator relays and to the battery hold relay 4|0, none of which relays is yet operated.

When the maximum transient operation time of the code relays TP, TN, RP and RN has elapsed, the enabler relay 404 releases. These four relays are the only code relays which are subject to extended transient operation. Relays MP and MN always have operate current and are not as subject to transient operation. Relay 404, released, starts relay 406 to release and connects battery, at the lower armature and front contact of relay 400, to the operating windings of all the multicontact translator relays, so that they are prepared to operate in accordance with the combination of code relays operated. Relay 4|0 also operates as a consequence of the release of relay 404 and locks in an obvious circuit. Relay 4|0 maintains battery on the multicontact relays even if the enabler relay 404 should be reoperated due to a short key closure.

With code relays MP, TN and RP operated as a consequence of the operation of key b at station A, translator relays MP2, TN 2 and RP2 now operate. Relay MP2 operates in a circuit extending from grounded battery, lower armature and front contact o relay 400, conductor 405, armature and right contact of relay 404, upper winding of relay MP2, back contact and outer lower armature of relay MN2, and upper armature and back contact of long closure relay 4|2, left contact and armature of code relay MP, conductor 40|, to ground at the front contact and Lipper armature of relay 400. Relay TN2 operates in a circuit extending from grounded battery,

lower armature and front contact of relay 400, conductor 405, armature and right contact of relay 404, upper winding of relay TN2, conductor i 90, contact and armature of code relay TN, conductors |21r and 400, left contact and armature of relay 405 (before this relay releases), left contact and armature of relay 402, conductor 40|, to ground by way of the front contact and upper armature of relay 400. Relay RPZ opera-tes in a circuit extending from grounded battery, lower armature and front contact of relay 400, con ductcr 405, armature and right contact of relay 404, upper winding of relay RPZ, conductor |95, contact and armature of code relay RP, concluc tors |27 and 409 and thence to ground at the front contact and upper armature of relay 400 as described in connection with the operation of relay TN2.

Relays MP2, 'IN2 and BP2, operated, lock to ground on the armature Aand left contact of relay 408 by way of their innermost lower armatures and front contacts. The ground on the armature of relay 400 also acts as a locking ground for relay 4 0. The operation of relays MP2, TN2 and R132 opens the operating circuit to slow-to-release relay 408.

When relay 400 operated, it established an obiious operating circuit for relay 200 (Fig. 3), by way cf the lower armature and back contact of relay 20 and conductor |60. Relay 200 operates in this circuit and connects the leads |41 to 45|, inclusive, to the windings of register relays 202 to 20G, inclusive, respectively, which constitute the iirst group of register relays.

With translator relays MP2, TN2 and RPZ and relay 200 of the decoder register circuit operated, a circuit is established for the operation of register relay 204. This circuit may be traced from grounded battery, lower winding of relay 204, conductor 20T, front contact and inner upper armature of relay 200, conductor |40, front contact and outer lower armature of relay TN2, front contact and outer lower armature of relay RP2, inner upper armature and front contact of relay MP2, conductor 40|, front contact and upper armature of relay 400, to ground.

Register relay 204, operated, locks in a circuit extending from grounded battery, upper winding of relay 204, front contact and upper armature of relay 204, conductor 208, winding of relay 20|, back contacts and outer upper armatures of relays 2|0, 220 and 230, conductor |46, to ground by way of the right contact and armature of relay 408. At its lower armature and front contact, relay 204 establishes an obvious circuit for the energization of a lamp signal at 2|?. Thus the character A of the telephone designation 21134-5678 is stored in the register for future use in controlling the automatic switches S, S-Z and S-3, which function to extend the call to the called station B.

When relay 408 released, it opened the locking circuit for the multicontact translator relays whereupon relays MP2, TNZ and RP2 release.

When relay 400 released, as hereinbefore described, it removed the ground connection to the armatures of relays RP, RN, TN and TP to prevent false operation of any multicontact translator relay due to an ending transient, and reconnected ground to the winding of relay 400 to hold this relay operated should the supervisory relay l2 release during reception of a digit. This ground is notapplied until the expiration of a predetermined interval of time after a digit is received, but the minimum release time of relay MN from their associated multicontact translator relays MP2 and MN2.

The translator circuit performs exactly in the saine manner as does the translator circuit shown in Fig. 4 of the aboVe-identied copending application. The several characteristic features of this circuit are set forth in detail in the copending application and do not require repetition here.

When relay 204 in the decoder register circuit operated as described above, its locking circuit was traced through the winding of relay 20|, so that relay 20| operated and locked to ground at the armature and front contact of relay 400 by way of conductor |60, and the uppermost front contact and armature of relay 20|. The ground is also carried through to the winding of relay 2 I 0 by way of the upper armature and two front contacts of relay 20| and the lower armature and back contact of relay 2||. Relay 2|0 operates and connects the leads |41 to |5|, inclusive, to the second group of register relays 2| 2 to 2|6, inclusive. At its lower amature relay 20| opens the circuit to relay 200 which relay releases its armatures.

The translator circuit has now been restored to its initial condition and the character A has been registered in the decoder register circuit, and the register has been prepared for the receipt of the next designation character D.

The subscriber at station A next actuates key c which in practice would be characterized by the symbols SDEFJ The operation of key c connects varistor to the tip conductor of the line L so that direct current of positive polarity traverses a path extending from ground, varistor I, closed contact of key c, tip conductor of the line to the corresponding brush and terminal of line finder F, sequence switch contacts |02, brush and corresponding rterminal of switch S|, tip conductor ||4, lower windings of code relays 'IP and TN, lead T, upper half of the secondary winding of transformer |0, lead M, lower windings of code relays MP and MN to ground. It will be observed that the circuit involving the lower half c-f the secondary winding of transformer I0, lead R, lower windings of code relays RN and RP is open in so far as having a varistor connected thereto. Current traversing the closed circuit traced above is in a direction such as to cause the operation of code relays MP and TP.

Code relay MP functions to prepare the translator circuit for operation in the same manner as it did when it operated in response to the actuation of key b at station A. It is believed unnecessary to repeat the detailed description of the operation of the translate-r circuit for each actuation of a key at the calling station.

With code relays MP and TP and register relay 2|0 operated, certain register relays will be operated to store the character D of the designation and the manner in which these relays operate will now be described.

Translator relay MP2 operates in a circuit extending from grounded battery, lower armature and front contact of relay 400, conductor 405, armature and right contact of relay 404, upper winding of relay MP2, back Contact and lower armature of relay MN 2, upper armature and back contact of relay 4|2, left contact and armature of code relay MP, conductor 40|, to ground by way of the front contact and upper armature of relay 400. Translator relay TPZ operates in a circuit extending from grounded battery, lower armature and front contact of relay 400, vconductor 405, armature and back contact of relay 4l4, upper winding of relay TF2, conductor |99, left contact and armature of code relay TP, con ductors |21 and 409, left Contact and armature of relay 405 (not yet released), left contact and armature of relay 402, conductor 40|, to ground by way of the front contact and upper armature of relay 400.

Relays MP2 and T132, operated, lock under control of relay 408. Register relay 2|3 now operates in a circuit extendingfrom grounded battery, lower winding o-f relay 2|3, conductor 228, contact 227 and its working spring of relay 2|0, conductor |49, back contact and innermost upper armature of relay RNZ, back contact and innermost upper contact of relay RP2, outermost upper armature and front contact of relay TF2, to gro'und at the front contact and upper armature of relay 400. Register relay 2| 4 also operates in a circuit extending from grounded battery, lower 'winding of relay 2|4, conductor 24|, front contact and innermost upper armature of relay 2|0, conductor |48, back Contact and outermost lower armature of relay RNZ, back contact and innermost upper armature of relay RP2, inner upper armature and front contact of relay MP2, conductor 40|, to ground by way of the front contact and upper armature of relay 400.

Register .relays 213 and 2|4, at their lower armatures establish obvious circuits for lamp signals at 229. Relays 2|3 and 2|4 lock in the same manner as did relay 204, previously described, and inthe locking circuitV therefor, relay 2| |v operates to cause relay 220 to operate, and to release relay 2|0. the leads |46 to |5|, inclusive, to the windings of relays 222 to 226, inclusive, respectively, of the third group of register relays, whose function it is to register and store the digit 4 of the telephone designation AD4-5678.

The equipment which functioned as a consequence of the operation of key c at substation A restores when the keyc is released, in a manner similar to that described after theregistration of the designation character A by the register relay 204. The telephone designation characters A and D are now stored in the decoder register and the equipmentis prepared to register the digit 4 in response to the actuation o f key d at the substation A.

When key d is actuated to cause the transmission to and registration at the central oice of thethird character 4 o-f the desired telephone designation, the varistor 3 is connected to the ring conductor of the line whereupon positive direct current impulses traverse a circuit which may be traced from ground, Varistor 3, closed contact of key d, ring conductor of the line L, corresponding terminal and brush of line iinder F, sequence switch contacts w3, brush I! and terminals of switch S-l, ring lead H3, left-hand windings of repeating coil RC, operating windings of code relays RP and RN, lead R, lower half of the secondary winding of transformer I0, lead M, operating windings of code relays MP and MN to ground. The direction cf the direct current in this circuit is such as to cause relays MP and RP to operate.

When the translator relays 402, 404 and 40B function, in the manner previously described, in response tothe operation of relay MP, the multicontact translator relays MP2 and ARPZ,k correspending to the code relays MP and RP, operate.

Relay 220, operated, connects Relay MP2 operates over the same circuit traced previously in connection with the actuation of keys b and c at station A. Relay RP2 operates in a circuit extending from grounded battery, lower armature and front contact of relay 499, conductor 495, armature and right contact of relay 494 (released) upper winding of relay RP2, conductor |95, contact and armature of code relay RP, conductors |21 and 409, left contact and armature of relay 496 (not yet released), left contact and armature of relay 492 (released), conductor 49|, to ground by way of the front contact and up-per armature of relay 499. RelaysI MP2 and RP2 lock to ground at the left contact and armature of relay 498.

Register relay 225 now operates in a circuit which may be traced from grounded battery, lower winding of relay 225, front contact and innermost lower armature of relay 229, conductor |41, back contacts and innermost upper armatures of code relays TP2 and TN2, outermost upper armature of relay RP2 in parallel with the inner upper armature and front contact of relay MP2 and their respective front contacts, conductor 49|, to ground by way of the front contact and upper armature of relay 490. Relay 225, by way of its lower armature and front contact, establishes an obvious circuit for the operation of a lamp signal at 239. At its upper armature and front contact, register relay 225 locks in a circuit which includes the winding of relay 24|. Relay 24| operates in this circuit causing the release of relay 229 and the operation of a relay, corresponding to relays 299, 2|9 and 229, associated with the next group of register relays.

The decoder register circuit shows. in detail only those groups of register relays which function to store the first three and the last characters of a telephone designation. It is to be understood that at least three, and possibly ve similar groups of register relays are interposed between the third and fourth groups illustrated. The broken lines to the left of Fig. 3 indicate the omission of the other groups of register relays of the decoder register circuit. The groups of register relays, not shown, function in the present case, in response to the successive actuations of keys e, f, and g to store the telephone designation digits 6 and 7.

Key h at the subscribers station A is actuated f,

for the transmission of the last digit, 8, of the desired designation whereupon a circuit is established which may be traced from ground, negatively poled varistor 2, closed contacts of key h,

tip conductor of line L, corresponding terminal Relay MN, operated, causes multicontact translator relay MN2 to operate while code relay TN causes translator relay TN2 to operate. The operating circuits for these relays need not be traced since they are substantially identical to the operating circuits for translator relays previously operated.

With relays MNZ and TN2 operated, decoder register relays 233, 234 and 235 operate.

Relay 233 operates in a circuit extending from grounded battery, lower winding of relay 233,

conductor 239, contacts 28| of relay 239, conductor |49, back contact and innermost upper armature of relay RN2, back contact and innermost upper armature of relay RP2, outermost upper armature and front contact of relay TN2, to ground by way of the front contact and upper armature of relay 499.

Relay 234 operates in a circuit extending from grounded battery, lower winding of relay 234, conductor 283, front Contact and innermost upper armature of relay 239, conductor |48, back contact and outermost lower armature of relay RN2, back contact and innermost upper armature of relay RP2, outermost upper armature and front contact of relay TN2, to ground by way of the front contact and upper armature of relay 499.

Relay 236 operates in a circuit extending from grounded battery, lower winding of relay 236, conductor 284, front Contact and middle lower armature of relay 239, conductor |50, inner upper armature and front Contact of relay MN2, outermost upper armature and front contact of relay TN2, to ground by way of the front contact and upper armature of relay 499.

At their lower armatures and front contacts, register relays 233, 234 and 236 establish obvious circuits for three of the lamp signals at 249. At their upper armatures, relays 233, 234 and 236 lock to ground on conductor |46. Thus the last Y telephone designation digit i8 is stored in the register. The release of key h at the subscribers station A restores the equipment to normal in a manner already described in connection with the release of key b.

The telephone designation ADL-5678 is now registered and stored in the decoder register circuit for use in controlling the operation of switches S, S-2, and S-3 which extend the call, originated at station A, to the station B.

The supervisory circuit and its operation will now be described.

Supervisory schemes in general require that the supervisory relay respond to switchhook operations at the subscribers stations. The relay is required to operate when the subset is on the line, that is, when the handset is removed from its switchhook or cradle support, and to release when the subset is disconnected from the line. In the present case, supervisory relay |2 (Fig. 1) is normally released. It receives operating current from the right-hand windings of repeating coil RC whose left-hand windings are included in the ring conductor I I3. When the subscriber at station A, initiated the call to station B, alternating current from the source P traversed the T, R and M leads of the code relay circuit over the closed loop at station A. The current traversing the lead R (Fig. i) obviously traverses the left-hand windings of repeating coil RC with the result that the current induced in the secondary windings of coil RC causes relay |2 to operate and to perform the functions heretofore ascribed to it, such as the operation of relay 499.

In any alternating current supervisory scheine there is introduced the difficulty caused by the flow of a charging current produced by the loop capacity. In Fig. 2, the line L extending from the central oce to the subscribers station A, Fig. l, is shown to include two capacities Cg to ground, and a metallic capacity Cm which appears between the two conductors of the line L. These capacities are inherent in the line L and are elective in causing the charging current from the source P to flow when the subset at station A is disconnected from the line L during signaling. This charging current obviously traverse the primary or left-hand winding of the repeating coil RC to Whose right-hand winding the rectifier X and the operating winding of the supervisory relay l2 are connected. Therefore, the operating winding of the relay l2 is subjected not only to alternating current from the source P when the line loop is closed, but also to a charging current caused by the loop capacitance when the line loop is open. It has been discovered that with a long enough loop this charging current obscures the change in impedance when the switchhook at the calling station A is operated and that the current in the supervisory relay is actually lower when the subset is on the line than when it is oi the line. What is required is that the current in the supervisory relay be larger when the subset is on, all lengths of line loop considered, than when the subset is 01T, all lengths of loop considered. Furthermore, the margin must be sumcient for a commercial relay adjustment. In the present invention this has been accomplished by utilizing a balancing voltage, which, in effect, cancels the voltage due to the average loop capacitance so that substantially no current flows in the supervisory relay when the subset is off the line.

This balancing voltage, in accordance with the present invention is supplied by means of the step-down transformer Il, which is supplied directly from the alternating current source P. The current due to this voltage lags the voltage of P because of the inductance of repeating coil RC vand therefore is 180 degrees out of phase with the charging current and is of such magnitude that the resultant current which flows in the supervisory relay, when the subset at station A is disconnected from the line L, is substantially zero for an average length of line loop and which, throughout the entire range of loop lengths, up to the maximum, never exceeds that value of current below which the supervisory relay is adjusted to release.

Fig. 6 illustrates, graphically, the effect of introducing a balancing voltage in the supervisory circuit of this invention. The horizontal straightlines O and R represent the operate and release current values, respectively, to which the super visory relay I2 is adjusted. For example, the line O indicates a current value above which the relay is adjusted to operate, while the line R indicates a current value below which the relay is adjusted to release. Obviously, the conditions of the line with Vwhich the relay is associated must be such that, when the substation A is disconnected from the line by the opening'of the switchhook contacts thereat, the current which 'traverses the operating windingof the supervisory relay must not exceed the release current value represented by the line R in Fig. 6, otherwise the relay would fail to release.

When the subset at station A is connected to the line L by the closure of the switchhook contacts, an operate current is caused to iiow over the line which functions to operate the supervisory relay l2'. This current is illustrated, graphically, by the downwardly sloping line a: of Fig. 6. This current varies from a maximum at zero loop to a minimum at the maximum length of loop, and'for the maximum length of loop this current obviously must exceed the operate current value to which the relay is adjusted, and which is represented by the line O.

The charging current produced by the loop capacity is represented by the upwardly sloping line y of Fig. 6. This current varies from zero, at zero loop, to a maximum at maximum loop length. This current, as hereinbefore indicated, is present in the loop when the subset at station A is off the line due to a disconnect. It will be observed that for certain lengths lof line, the current due to the line capacity is greater in value than the operate current :c so that the'current in the relay l2 would be greater when the subset at station A was ofi" the line than when it was on the line. As hereinbefore indicated, the converse is desired.

It will also be observed that for certain lengths of line, the current due to line capacitance exceeds the release current value R to which the relay l2 is adjusted to release. The supervisory relay, therefore, would fail to release under these circumstances and would remain operated regardless of whether or not the line was closed at the substation A.

Balancing out the Voltage due to the line capacitance and which produces the current y shown in Fig. 6, results in an open circuit (subset disconnected) current whose graph is shown at e in Fig. 6. It will be noted that this current graph is always below the release current line R, indicating that the open circuit current to which the supervisory relay is subject never exceeds the current value below which the relay is adjusted to release.

The currents due to the loop capacitance voltage and to the Voltage supplied by transformer ll are rectied at X and impressed upon the operating winding of relay I2. Under the condition in which the subset at station A is oi the line, that is, due to a disconnect, the supervisory relay I2 is subject to current produced by the voltage caused by the line capacitance and also to current produced by the Voltage supplied by transformer Il, and since, by design, these currents are in phase opposition, they effectively tend to cancel out, and the net current owing in the relay winding, due to these two sources is minimized. During signaling, when the oirnormal contact Q is opened, the relay I2 may or may not release depending on the digit key depressed and on the length of loop, but the slow release relay 460, operated by relay I2 between digits, is kept operated by relay 466 as already described.

What is claimed is:

1. In a telephone system, a calling station, a central oce, a source of single-phase alternating current at said central oilice, a transformer, a two-wire code receiving circuit terminating in the secondary winding of said transformer, said source of current being connected to the primary winding of said transformer, means responsive to the initiation of a call at said station for extending said code receiving circuit to said station, means at said station for translating the current from said source into direct current impulses and causing them to be transmitted to said code receiving circuit, and means selectively responsive to the transmitted impulses comprising a pair of oppositely poled polarized relays in each wire of said code receiving circuit and in a branch circuit extending from a center tap on the secondary winding of said transformer to ground.

2. In a code signaling system, a sending station, a central oiice, a source of single-phase alternating current at said central oice, a code receiving circuit at said central oflice comprising a pair of conductors connected to the line conductors of said sending station, a transformer having its primary winding connected to said source of alternating current and a secondary winding connected across the conductors of said code receiving circuit, a branch circuit extending from a center tap of the transformer secondary winding, and a pair of polarized relays serially included in each of the conductors of said code receiving circuit and in the said branch circuit, a plurality of current rectifying devices at said sending station, and means at said sending station for connecting said rectifying devices in various combinations to the line conductors of said station Ato cause rectiiied current impulses to be transmitted over th-e conductors of said code receiving circuit to selectively operate one of each of said pairs of polarized relays.

3. In a code signaling system, a sending station, a central oii'ice, a source of single-phase alternating current at said central oce, a two- Wire code receiving circuit connected to the line conductors of said sending station, a transformer connecting said source of current to said code receiving circuit, a pair of polarized relays serially included in one wire of said code receiving circuit, a pair of polarized relays serially included in the other wire of said co-de receiving circuit, :a branch circuit extending from a center tap on the secondary winding of said transformer to ground, a pair` of polarized relays in said branch circuit, a plurality of varistors at said sending lstation having normally grounded and ungrounded terminals, and switching means at said station for selectively connecting the ungrounded terminals of certain of said varistors to the line conductors of said sending station in such a manner, that current from said source is rectied by one only of said varistors and caused to flow over both wires oi said code receiving circuit and over said branch circuit to cause the operation of a relay of each of said pairs of polarized relays.

4. In a code signaling system, a sending station, a source of single-phase alternating current, a two-wire code receiving circuit, a transformer coupling said source of current to said code receiving circuit, a branch circuit extending from a center tap on the secondary winding of said transformer to ground, signaling devices included in each wire of said code receiving circuit and in said branch circuit, and switching means at said sending station for connecting either of the wires of said code receiving circuit to ground through a varistor and for simultaneously shorting the other wire of said code receiving circuit to ground, whereby direct current is caused to ow over one wire of said code receiving circuit and in parallel over the other wire and said branch circuit to cause certain of said signaling devices to operate.

5. In a code signaling system, a sending station, a source of single-phase alternating current, a two-wire code receiving circuit connected to the line wires of said sending station, a transformer having its primary winding connectedacross said source of current and a secondary winding :terminating Vthe wires of said code receiving circuit, direct current responsive devices included in each wire of said code receiving circuit, a branch circuit extending from a center tap on the secondary of said transformer to ground, direct current responsive devices included in said branch circuit, two pairs of varistors at said sending station. the varistors of each pair having an opposite terminal connected to ground, and switching means at said sending station for connecting the ungrounded terminal of a varistor of the rst of said pairs to one line wire of the sending station and the ungrounded terminals of both varistors of the second of said pairs to the other line wire, whereby direct current of a polarity determined by the connected varistor of the first pair traverses both wires of said code receiving circuit and said branch circuit to cause the operation of a certain combination of said direct current responsive devices.

6. In a code signaling system, a sending station, a receiving station, a single-phase source of power at said receiving station, a transformer at said receiving station, a two-wire code receiving circuit terminating in the secondary of said transformer whose primary is connected across said source of power, a branch circuit extending between a center tap on the secondary of said transformer and ground, signaling devices included in each wire of said code receiving circuit and in said branch circuit, a plurality of oppositely poled varistors at said sending station each having one terminal grounded, and switching means at said sending station for connecting the ungrounded terminal of one of said varistors to one wire of said code receiving circuit and simultaneously connecting the ungrounded terminals of two oppositely poled varistors to the other wire of said code receiving circuit, whereby the said other wire of said code receiving circuit is eiectively grounded at said sending station and direct current of a polarity determined by the varistor connected to the said one wire of said code receiving circuit is caused to traverse each of the two wires of said code receiving circuit and said branch circuit, to effect the selective operation of signaling devices included therein.

7. In a telephone system, a calling station, a central oiice, a source of alternating current at said central oilice, a line connection interconnecting said station and said central office and supplied with current from said source, a supervisory relay, means for inductively connecting the winding of said relay to said line connection, and other means for connecting the winding of said relay directly to said source of alternating current.

8. In a telephone system, a calling station, ,a central oince, a source of alternating current ,at said central oiilce, a line connection interconnecting said station and said central office and supplied with current from said source, a repeating coil having a primary winding included in said line connection, a transformer having a primary winding connected to said source, and a series connection including the secondary windings of said repeating coil and said transformer and the winding of said supervisory relay.

9. In a telephone system, a subscribers stationy a central office, a source of alternating current at said central oice, a line connection interconnecting said station and said central oiiice and supplied with current from said s'ource, a supervisory relay, and separate means for connecting said supervisory relay to said line connection and to said source of current comprising a repeating coil and a transformer having their secondary windings included in series with the operating winding of said relay.

l0. In a telephone system, a subscribers station, a central oiilce, a two-wire circuit interconnecting said station and said central oiice, a source of alternating current connected to said two-wire circuit, a supervisory relay, a repeating coil having its primary winding included in one wire of said two-wire circuit, and a stepdown transformer having its primary winding supplied directly from said source and its secondary winding included in a series circuit including the secondary winding of said repeating coil and the operating winding of said relay.

1l. In a, telephone system, a subscribers station, a central oice, a line loop interconnecting said station and said central oilice, a source of alternating current connected to said line loop, a supervisory relay, means for connecting said relay to said line loop so as to subject it to a voltage produced by current caused to ow in said loop when the subset at said station is connected to said loop, and auxiliary means for subjecting said relay to a voltage other than the voltage produced by the flow of current in said loop.

12. In a telephone system, a subscribers station, a central oice, a loop circuit extending from said station to said central oice, a source of alternating current, said loop circuit having an inherent capacitance characteristic wherebycurrent from said source due to the loop capacitance is caused to flow in said loop circuit, a supervisory relay, means connecting said relay to said loop circuit so as to subject said relay to a voltage commensurate with the current in said loop caused by the capacitance thereof, and means for subjecting said relay to a voltage of such magnitude and phase with respect to the voltage produced by the loop capacitance current that the two voltages substantially cancel each other.

13. In a telephone system, a subscribers line, a station thereon, a two-wire supply circuit connected to said line and having a source of alternating current connected thereto, said line constituting a loop circuit having an inherent capacitance characteristic which causes a charging current to flow in said loop from said source, a repeating coil having its primary Winding included in said loop so as to be traversed by the charging current, a supervisory relay having its operating winding connected to the secondary winding of said repeating coil so as to be subject to the voltage set up therein by the flow of charging current in the primary thereof, and means for superimposing on said relay a voltage which, for average loop length is substantially equal in magnitude and in phase opposition to the voltage produced by the charging current owing in the primary winding of said repeating coil.

14. In a telephone supervisory signaling system, a telephone line, a two-wire circuit adapted to be connected to said telephone line, a source of alternating current connected to said twowire circuit, said telephone line having an inherent capacitance characteristic such as to cause a charging current to iiow therein from said source, a supervisory relay having its operating winding inductively connected to said twowire circuit and therefore subject to the charging current flowing in said line, and means for balancing the effect of the charging current on said supervisory relay comprising transformer means for connecting the operating winding of said supervisory relay directly to said source of alternating current.

15. In a telephone signaling system, a subscribers station, a loop circuit having an inherent capacity characteristic, extending from said station to a central oflice, a source of alternating current associated with said loop circuit, said loop circuit being normally subjected to a charging current produced by the inherent capacity characteristic thereof and its association with said source of alternating current, a supervisory relay included in said loop circuit, and means for subjecting said relay to a current of such phase and magnitude with respect to the `charging current of said loop as to substantially cancel it whereby the current in said relay when said station is disconnected from said loop is substantially zero.

STUART M. BABCOCK. MASON A. LOGAN, 

